The present invention relates to an intrusion detector and more particularly to an intrusion detection having a housing and an infrared detection portion disposed therein, comprising an infrared sensor, a detector window provided in the housing wall for the passage of infrared radiation from the external space to the infrared sensor, a means for focusing the external infrared radiation transmitted through the detector window onto the infrared sensor, and a sabotage surveillance device including an infrared transmitter and an infrared receiver.
Sabotage surveillance devices, which are also referred to as anti-mask devices are described, for example, in EP-A-0 186 226, in EP-A-0 499 177 and in EP-A-0 556 898. These devices serve to detect the two types of detector masking, i.e. detector masking at a certain distance from the detector window, which distance may be only small, and the direct masking of the detector window, for example, by masking the window with a foil or spraying it with an infrared-opaque spray, such as, for example, paint spray. The first type of masking is referred to as remote masking and the second type as spray masking. Remote masking is understood to mean a masking effected at a distance from a few millimeters up to a maximum of about 15 cm.
Changes immediately in front of a detector, such as remote masking, generally affect the reflection of the radiation emitted by the infrared transmitter of the anti-sabotage device from the detector window onto the infrared receiver and cause a change in the radiation received by the infrared receiver. To detect changes in the transmission properties of the detector window, infrared radiation is emitted in the direction of the detector and the radiation passing through the detector window or reflected thereby is measured. To evaluate the signals of the anti-sabotage device, the signals of the infrared receiver are compared with threshold values or reference values or, generally, voltage values that have to be exceeded or not reached and have to be maintained over a certain period of time.
The known sabotage surveillance devices are constructed as single-channel or two-channel systems. In the case of two-channel systems, such as, for example, the device described in EPA-0 186 226, a first infrared transmitter that is disposed in the interior of the detector emits infrared radiation into the surveillance space in front of the detector and a first receiver measures the radiation reflected from the surveillance space. A second infrared transmitter disposed on the outside of the detector emits radiation through the detector window onto a second receiver that measures the incident radiation of the second transmitter. The first transmitter and the first receiver form a channel for surveying sabotage attempts of the remote masking type and the second transmitter and the second receiver form a channel for surveying sabotage attempts of the spray masking type.
In the single-channel system described in EP-A-0 499 177, the sabotage surveillance device contains only one infrared transmitter and only one infrared receiver, the transmitter being disposed on the outside of the detector and the receiver in the interior. The transmitter transmits infrared radiation into the surveillance space in front of the detector and through the detector window onto the receiver. A similar single-channel system is described in EP-A-0 556 898.
Common to all known sabotage surveillance devices, regardless of whether they are designed as a single-channel system or as a two-channel system, is the fact that an element of the device is disposed on the outside of the detector. This arrangement influences, to a certain extent, the design of the detector housing because a protruding section for receiving the infrared transmitter must be present on the housing opposite the detector window which substantially influences the external appearance of the detector.
In the anti-masking device described in EP-A-0 772 171, an optical diffraction grating structure is mounted on the outside of the detector window that focuses light emitted by an infrared transmitter onto the optical receiver. In the event of sabotage as a result of spraying the detector window, the focusing action of the optical diffraction grating structure is affected so as to reduce the light intensity focused onto the infrared detector. Although the infrared transmitter is disposed in the interior of the detector in this device, the optical diffraction grating structure is located externally. An important draw back of this design is that particles contained in the air of the space under surveillance, for example smoke particles or soot particles or even grease particles, become deposited on the grating structure, as well as the detector window, which over time discolors and even affects the infrared-radiation transmission properties. This structure may constitute a technical disadvantage which impairs the serviceability of the detector. The potential discoloration of the diffraction grating on the detector window may also constitute an aesthetic disadvantage. In addition, the optical diffraction grating structure is not suitable for detecting remote masking.
An object of the present invention is thus to provide an intrusion detector having a sabotage surveillance device that detects both types of sabotage, namely remote masking and spray masking. Preferably, such sabotage can be detected in the so-called real-time mode. Real-time mode is understood as meaning a method in which only sufficiently large and sufficiently stable changes trigger a sabotage alarm that is automatically withdrawn if the signals return to the normal state. Although this mode responds more slowly than the second known method, the so-called proximity latch mode, it has the advantage of automatic alarm cancellation. In addition, the intrusion detector upgrades the sabotage surveillance device that neither restricts the creative range for the design of the detector housing nor results in impairments of the functional reliability of the external appearance of a detector equipped with such a device
An improvement realized by the present invention is that the infrared transmitter and the infrared receiver are both disposed within the housing, with the detector window being substantially transparent to radiation emitted by the infrared transmitter. The present arrangement obviates the need for an externally mounted diffraction grating on the detector window. Sabotage of the detector is kept under surveillance by measuring the proportion of the said radiation reflected onto the infrared receiver from the inside surface of the detector window and that reflected from the surrounding space.
The arrangement of transmitter and receiver inside of the detector window affords not only aesthetic aspects in the design of the detector housing, but also lessens the risk of excessive particle deposits on the detector window which tends to accumulate in external defraction gratings. And perhaps even more important is the fact that the detector has a sabotage surveillance device is not detectable from the external appearance of the device.
A further aspect of the invention is that the infrared receiver of the sabotage detector device is capable of compensating for extraneous light passing through the detector window.
Yet another aspect of the present invention is that the means for focusing the infrared radiation from the external space which passes through the detector window is formed by a base layer of dark material and a mirror having a reflection layer applied to the base layer. The reflection layer is transparent to interfering radiation below the typical wavelength range of human thermal radiation and yet highly capable of reflecting radiation within the wavelength range of human thermal radiation. In this content, xe2x80x9cdark materialxe2x80x9d means a material that absorbs well below a wavelength of about 4 xcexcm. The reflection layer is substantially transparent in the visible range and transmits infrared radiation of shorter wavelengths, preferably below 4-7 xcexcm so that the latter can reach the dark base layer, where it is absorbed. Furthermore, the layer of dark material has the effect that as little interfering light as possible falls on the sensor and on the infrared receiver, which is important in order for the detector to be able to detect both types of sabotage, remote masking and spray masking, in the real-time mode.
In a preferred embodiment of the intrusion detector according to the present invention the detector includes an ancillary device for detecting an intruder comprising an additional transmitter and an additional receiver. The signal of the additional receiver has two frequency ranges, one of which is typical of movements in the space under surveillance and the other is typical of a masking affect on the detector. A common evaluation circuit is provided for the ancillary detection means and the infrared detector. The ancillary device is preferably an ultrasonic device having an ultrasonic transmitter and an ultrasonic receiver or a microwave section having a microwave transmitter and a microwave receiver.
In a further preferred embodiment of the intrusion detector according to the invention the evaluation circuit has a passive infrared (xe2x80x9cPIRxe2x80x9d) channel connected downstream of the infrared sensor, an anti-mask channel connected downstream of the infrared receiver and an ultrasound (xe2x80x9cUSxe2x80x9d) channel that is connected downstream of the second receiver and a US anti-mask channel. The evaluation circuit may also have a combining stage connected to the outputs of the aforesaid channels for the combined evaluation of the signals of the channels.
The invention is explained in greater detail below by reference to an exemplary embodiment shown in the drawings.